1. Field of the Invention
The present invention relates to a measuring instrument for determining displacements, preferably those in the soil, rock, foundation soil or construction materials, which include first and second hinge-coupled elongate, substantially rigid measuring elements, and a measuring device for determining changes of position of the first and second measuring elements relative to each other, brought about by pivoting of the first measuring element relative to the second measuring element, wherein the measuring device includes a flexible element interconnecting the first and second measuring elements and a mechanism for determining the curvature or bending of the flexible element which occurs by pivoting of the first measuring element relative to the second measuring element, as a measure of relative displacement in the foundation soil or construction.
2. Discussion of the Background
Measuring instruments of this type are known from CH-PS 636698 and CH-PS 675910, the use of which make it possible to determine displacements occurring in the foundation soil, constructions or other materials, which have the effect of changing the relative position of two measuring elements in a direction running transversely to the longitudinal axis of the instrument. The measuring device known from the first-named patent specification exhibits an inductive length measuring system, by which, when a measuring element is pivoted out of the reference direction defined by the aligned longitudinal axes of the measuring elements, the size of the deviation from this reference direction is determined. The length measuring system which is used has the disadvantage that it is prone to producing measuring errors. The measuring method using the known instrument is expensive, since the instrument exhibits a large diameter such that a correspondingly large borehole diameter is required when determining displacements in the foundation soil. The instrument is also awkward to use and is not watertight, so that it cannot be employed in a water-filled borehole.
In the case of the instrument known from the second-named patent specification, there is disposed in the one measuring element a laser light transmitter, the emitted laser beam of which, aligned with the longitudinal axis of this measuring element, strikes a receiver which is disposed in the other measuring element and has a photo-detector device, so that if, in the foundation soil, a transverse displacement has occurred which results in pivoting of the measuring elements relative to each other, the laser beam striking the photo-detector device is deflected out of its zero position, the extent of this deflection being determinable by an evaluation circuit. The measuring method is accurate and also allows measurements in two mutually perpendicular lanes, but it is expensive since the electronics are very complex. The measuring range is relatively small and the instrument, due to the optical components present, is sensitive to humidity so that, for this reason and also for the reason of cost, a fixed installation of the instrument into a borehole is out of the question.
In addition, "deflectometers" are known (e.g. the INTERFELS-EASTMAN system), in which a measuring member anchored in a borehole is disposed between two endpieces, which measuring member is cardanically connected, by means of spacing tubes, to the two endpieces. A tensioned wire, extending from the one to the other endpiece and through the tubes and measuring member, is precisely mounted, by means of precision cutters disposed in the endpieces and in the measuring member, and an inductive path sensor is disposed in the measuring member such that it is able to measure a change in the position of the measuring wire resulting from a transverse displacement. In the case of a "chain-deflectometer", this extended embodiment contains a plurality of consecutive measuring embers. The instrument possesses a large diameter, requiring correspondingly large borehole diameters, so that the measuring method is relatively expensive to use. Moreover, the measurement of displacements is only possible in one plane. Any slackening of the tension of the measuring wire affects the measuring accuracy.
In addition, it has also been proposed (e.g. Publication: "Rock Mechanics Suppl. 8" (pages 349-367, Springer Verlag 1979)) to fit wire strain gages in the interior of an aluminum tube encased in concrete in a borehole, in order to measure the bending elongations in the aluminum tube caused by transverse displacements in the foundation soil. However, only a low level of measuring accuracy is thereby obtained, the measuring results also being erroneous if there is a strong shearing deformation between two neighboring measuring points. Fitting of wire strain gages in the interior of a tube is complicated and costly and there is no facility for calibration under this method.